Cathode ray tube of the index type

ABSTRACT

The invention is related to a cathode ray tube of the index type wherein the tracking structure allows the type to be used in progressive scan mode. The tracking structure comprises tracking elements ( 16,18 ) of a first kind and a second kind for generating a first response signal (S 1 ) and a second response signal (S 2 ), respectively, when hit by an electron beam of the tube, the first and the second response signals for determination of a positioning signal, and the tracking elements ( 16,18 ) parallel to the phosphor elements ( 20,20′,20 ″) whereby each phosphor element is flanked on either side by a tracking element of the first kind ( 16 ) and a tracking element of the second kind ( 18 ), respectively, except for each phosphor element of the third (B) color ( 2040 ″) of each third set ( 330 ), whereby each side of said phosphor element of the third (B) color is flanked by tracking elements of the same kind. This tracking structure has the advantage that the index tube has no noticeable flicker if operated in interlaced scan mode.

The invention relates to a cathode ray tube of the index type.

Cathode ray tubes of the index type operate without a shadow mask. Aninner screen is provided with phosphor elements that extend in adirection, preferably the horizontal direction. Each phosphor element isflanked, preferably above and below, by tracking elements belonging to atracking structure. Proper landing of the electron beam on the desiredphosphor elements (conventionally the phosphors generate red, green andblue light) is assured by using a correction signal that containsinformation about the deviation of the electron beam from the idealposition on the phosphor element. The tracking elements comprisephosphors that excite light when being hit by the electron beam. Thephosphors of the tracking elements positioned above the phosphorelements excite with a first wavelength and the phosphors of thetracking elements positioned below the phosphor elements excite with asecond wavelength when being hit by the electron beam. The excited lightis detected by two photo-detectors, a first detector being sensitive tothe first wavelength, and a second detector being sensitive to thesecond wavelength. The signals from the two detectors are used to derivethe correction signal for correcting the position of the electron beamin case it deviates from the ideal path over the phosphor element bymeans of a feedback loop that steers the beams to the center of thephosphor lines, thereby avoiding color errors.

It is a problem of the conventional index tube that the tube can only beused if driven in so-called progressive scan mode. It is desired thatthe tube may be used in so-called interlaced scan mode, which is alsothe mode of driving conventional cathode ray tubes. However, if theconventional index tube is driven in interlaced scan mode this resultsin a displayed image that has an unacceptable level of flicker.

It is an object of the invention to provide a cathode ray tube of theindex type that can be driven in interlaced scan mode without any imageflicker. To this end the cathode ray tube according to the invention isdefined by independent claim 1. The dependent claims describeadvantageous embodiments of the invention.

The invention is based on the insight that the tracking structure of theconventional index tube provides an error signal that alternates in signbetween successively scanned phosphor lines. To reduce disturbances ofthe error signal it is necessary in the conventional index tube toaverage the error signal over the last two lines. For this averaging tobe effective the relevant part of the error signal should change signfrom phosphor line to phosphor line. If the conventional index tubestructure is used in a progressive mode this sign change takes place.However, when used in interlaced mode, the sign change is absent and thetracking will result in a noticeable screen flicker.

In the tracking structure according to the invention the error signalalternates between successively scanned lines and the disturbances canbe suppressed. Consequently no flicker is present if the tube is used inprogressive mode.

These and other objects of the invention will be apparent from andelucidated with reference to the embodiments described hereinafter.

In the drawings:

FIG. 1 shows a conventional CRT of the index type,

FIG. 2 shows a tracking structure of a conventional index type CRT,

FIG. 3 shows an embodiment of a tracking structure according to theinvention,

FIG. 4 shows a second embodiment of a tracking structure according tothe invention, and

FIG. 5 shows a third embodiment of a tracking structure according to theinvention.

The figures are not drawn to scale. In the figures, like referencenumerals generally refer to like parts.

In FIG. 1, a display apparatus of a conventional index type colorcathode ray tube 1 having an evacuated envelope 2 comprising a displaywindow 3, a cone 4 and a neck 5. The neck 5 accommodates an electron gun6 for generating electron beams 7, 8 and 9 extending, in thisembodiment, in one plane, the in-line plane. In the in-planeconfiguration, there are two side beams and one central electron beam. Adisplay screen 10 comprises a plurality of red, green andblue-luminescing phosphor elements. On their way to the display screen10, the electron beams 7, 8 and 9 are deflected across the displayscreen 10 by means of a deflection unit 11.

The tube further comprises an element 12 from which a first responsesignal S1 and second response signal S2 are fed to a deflectioncorrection generator 730 that generates a deflection correction signal ƒbased on the two response signals. A Composite Video Baseband Signal(CVBS, or a similar video input signal) is applied to a DeflectionSignal Generator (DSG) for generating a deflection signal, a PictureSignal Processor (PSP) for generating a picture signal for gun 6 and aGrid Signal Generator (GSG) for generating a signal for grid elements14. The deflection signal ƒ is combined with the deflection signal fromthe DSG and used as deflection signal 732 for the deflection unit 11.

For a three-electron beam the feedback mechanism comprises twosubsystems, a slow and fast feedback loop. The fast feedback loopcorrects disturbances that effect the landing position of all threebeams simultaneously. In general these disturbances come from outsidethe tube and occur at relatively high frequencies. Examples are theelectromagnetic fields induced by transformers of halogen lamps or thefields induced by mobile phones.

The fast loop corrects the landing position of all three beams by meansof a magnetic dipole. This loop is able to correct for verticaldisplacements of the order of one phosphor line, larger displacementswill result in tracking errors (i.e. tracking on the wrong phosphorline).

The slow feedback loop corrects (slow) disturbances that can effect thelanding position of the individual beams. Examples of such disturbancesare the astigmatism and coma errors from the deflection yoke. Theseerrors change in time due to heating of the yoke. The time scales atwhich these changes occur are typically of the order of tens of minutes.The slow feedback loop corrects the landing position of the individualbeams by means of coils that generate magnetic dipole-, quadrupole- andsextupole-fields.

The conventional cathode ray tube (hence the non-index CRT) is operatedin so-called interlaced scan mode. In Europe a complete picture to bebuild up on the screen uses 625 picture lines (here phosphor triplets),which run from left to right and from top to bottom. A ‘fresh’ pictureis written on the screen 25 times every second. First all ‘odd’ linesare scanned (written) to the bottom of the picture, then a quick jumpback to the top of the screen takes place after which the intervening(‘even’) lines are scanned in the same way. The ivision into an even andan uneven half-frame is called interlaced scanning. Due to the way heerror signal is constructed the conventional index tube cannot beoperated in the above-described interlaced scan mode. Conventional indextubes are therefore operated in a progressive mode, i.e. the displayedpicture is built-up of a full frame that is obtained by progressivelyscanning all lines.

FIG. 2 shows a detail of a tracking structure of a conventional cathoderay tube of the index type. The tracking structure is located on aninner surface of the screen 10, which has phosphor elements 20,20′,20″.Tracking elements 16 and 18 extend parallel (preferably horizontally,i.e. parallel to the x-axis) to the phosphor elements. Tracking elements16 and 18 are positioned adjacent to phosphor element 20.

Simultaneously, the three electron beams 7,8,9 are scanned over thephosphor elements. Each beam scans over a phosphor element emittingeither red light (in case of electron beam 7), green light (electronbeam 8) or blue light (electron beam 9), thus forming a pixel element.For reasons of conciseness, the terms red, green and blue electron beam,respectively, will be used. When the electron beam, for example electronbeam 8, hits the tracking element 16, which is located above thephosphor element 20, light of a first wavelength is emitted andregistered by a first photo-detector located on or in the tube. Whereaswhen electron beam 8 hits the tracking element 18 located below thephosphor element 20 light of a second wavelength is emitted andregistered by a second photo-detector. Electron beam 8 impinging onphosphor element 20 will also impinge on tracking elements 16 and 18.When the electron beam evenly impinges on tracking elements 16 and 18,there will be no difference in response signals from the trackingelements. When the electron beam is shifted upwards or downwards, moreelectrons will impinge on one of the tracking elements than on the otherand a difference in response signals will occur. This difference can bemeasured and used for correcting the position of the electron beam 8with respect to phosphor element 20.

The beams are separated from each other by one phosphor line. This isdone to obtain a difference signal S1-S2 that has the same sign for allthree beams, which allows to combine fast tracking of the averageposition of the three beams with a slow tracking of the position of theside beams with respect to the center beam. This way of tracking thebeams is disclosed in WO02/093612.

In the conventional index tube a vertical average of the tracking signalis used to steer the beams over the right phosphor track. Thedisturbances are low frequent compared to the line frequency. Thereforethe error signal on the j−1^(th) line is a good prediction of the errorsignal on the j^(th) line.

However, there is one complication with interlaced tubes: the errorsignal has a symmetric contribution. So, for odd numbered lines theerror signal can be written as:ε₁ =αΔy+βα is a constant, Δy the average vertical displacement of the beams and βa symmetric disturbance. This disturbance is the result of asymmetriesin the amplifiers, detectors, etc. For the even lines the error signalreads:ε₂ =−αΔy+β

To remove the symmetric disturbance the error signal of two lines aresubtracted and used as an error signal for the feedback:$\hat{ɛ} = {{\frac{1}{2}\left( {ɛ_{1} - ɛ_{2}} \right)} = {{\alpha\Delta}\quad y}}$(Or the error signal of successive lines are used to get a larger timeaverage and to suppress noise).

For a progressive index tube the above indicated averaging works.However, for an interlaced tube the averaging is not possible. For theodd (even) frames all error signals have the sane sign. So, there is noway in which the symmetric disturbance can be removed. Experiments haveshown that a dramatic flicker is obtained if the symmetric disturbanceis not removed.

FIG. 3 shows an embodiment of a tracking structure according to theinvention.

The phosphor elements are grouped in sets of three 310,320,330. Eachphosphor element 2000,2000′,2000″,2020,2020′,2020″,2040,2040′ is flankedon either side by a tracking element of the first kind 16 and a trackingelement of the second kind 18, respectively, except for each thirdphosphor element 2040″ of each third set 330, whereby each side of saidthird phosphor element is flanked by tracking elements of the same kind,in this specific example a tracking element a tracking element of thefirst kind 16.

The tracking structure according to the invention is periodic over threephosphor triplets. If used in a progressive mode, the third and sixthline do not yield any useful feedback signal. For the first to sixthline the error signals read:ε₁=αΔy+βε₂=−αΔy+βε₄=αΔy+βε₅=−αΔy+βAn estimation of the tracking signal for the third and the sixth linecan be obtained by extrapolation.

In an interlaced mode the second and the third line are not observablein the odd and even frames respectively. So, in the even frame the errorsignals read:ε₁=αΔy+βε₃=−αΔy+βand in the odd frames:ε₁′=αΔy+βε₂′=αΔy+βHere as well by a proper linear combination of previous error signals anextrapolation to estimate the missing lines can be made.

FIG. 4 shows a further embodiment of a tracking structure according tothe invention. In this embodiment a subset of the tracking elements161,181 have gaps 30,30′ for deriving an additional positioning signalfor positioning the electron beam. By providing a subset of the trackingelements with gaps a temporary interruption of the correction signal isgenerated, when the electron beam is at a well defined position of thescreen. This interruption is additionally used to control the electronbeam, in particular during the start-up period of the television set.

FIG. 5 shows a further advantageous embodiment of the invention. Gaps30,30′ of m adjacent phosphor elements form part of a first column 42,and gaps 31,31′ of n adjacent phosphor elements form part of a secondcolumn 44. Both columns extend in a direction perpendicular to thetracking elements. The first 42 and the second column 44 are positionedadjacent to each other. In the example shown in FIG. 5 m is equal tonine and n is equal to five, while the first and the second column arepositioned symmetrically with respect to each other, i.e. centers of thecolumns are positioned on the same phosphor element. In view of itsshape, this structure is also called T-structure.

The purpose of the first column (which is scanned first in time, asscanning in this example takes place from left to right) is to provide astart-of-structure signal, that can be detected even when themacroscopic correction is completely wrong.

If the tracking of the electron beams is good, only the scanning of fourscan lines is influenced by this structure (in one scan three beams arescanned along three phosphor lines, four of these scans are affected).The three beams at positions 100 and 600 will miss the structure. Thethree beams at positions 200 and 500 will be negatively influenced bythe structure; in the first column 42 of the structure. At bothpositions the green beam (i.e. the middle) will hit only one of the twotracking elements, so the tracking signal can temporarily not be used.

At positions 300 and 400 the electron beams hit the first column 42 ofthe structure; there will be no tracking signal at all. This conditionallows a reliable detection of the start of the structure, even whenlife video is shown. In the second column 44, the tracking signal willcome from only the red beam in case of position 300, and only from theblue beam in case of position 400.

The embodiment of the invention shown in FIG. 5 has additionaladvantages related to raster correction, convergence and focus of theelectron beam. These advantages will be explained hereinafter.

Raster Correction

If the tracking is wrong, the first column of the structure can still bedetected quite easily. Therefore, getting the macroscopic trackingaspects of the tracking right is greatly simplified by the presence ofthis structure. During the calibration phase, a uniform green testpattern can be displayed. The photo-detectors will generate a signal inwhich the structures are easily detectable. Pattern matching algorithmsproduce position information with a resolution better than one phosphorline. This gives immediate absolute horizontal and vertical positioninformation about the scanned raster, and thus control settings of thetube like width, height, linearity etc can easily be adjusted.

As information is required from all screen areas, a large number ofthese T-structures is required. They can be placed on the positions of ax by y-matrix that covers the whole screen. The ideal number andhorizontal width of these positions depend on a large number of aspects,such as: bandwidth of the optical detection mechanism, horizontal spotsize and maximum correctable misalignment, etc.

A matrix of 9 by 9 T-structures has proved to be a practical value.

Convergence

Convergence can be measured at the position of the T-structure, withoutspecial requirements on the video contents, other than that it shouldnot be completely black. Assuming that the macroscopic position iscorrect, and that the microscopic tracking keeps the combined threebeams on track, the structures provide convergence information in thesecond column, which allow the red and blue side beams to be adjusted sothat the vertical distance between the three beams is correct.

In the area outside the structure, the measured tracking signal is aweighted average of the contribution of the red, green and blue beams.The weight is dependent of the video contents, and therefore notcontrolled by the tracking system itself. However, in the second columnof the structure, the beams at position 300 in FIG. 4 will generate atracking signal which only depends on the red beam, and at position 400only depends on the blue beam. If there is a difference between theweighted signal from red, green and blue, and the signal that is onlydependent on red, it is clear that the red side beam is off-track. Howmuch off-track can not be determined, as the video information candiffer between these two measurements. But the sign of the differencesignal is always correct, and can be used to adjust the red side beam insmall steps into the proper direction. This is a proper method, asconvergence errors have a slow drift behavior. In order to preventimages that do not contain red at the position of the structure, theadjustments must not be performed when the measured signal drops below aspecific threshold value. The same method is to be applied to the bluebeam.

The advantage of this method is that it can be performed duringoperation of the tube, not only within the start-up phase of thetelevision set, and without visible interference.

Focus

A global focus adjustment can be performed by minimizing the signallevel that is generated by the two tracking phosphors: the better thespot size in the vertical direction, the lower the number of electronshitting tracking phosphors. In the conventional tracking structure, onlythe combined focus can be judged, and the contributions per color dependon the video content shown. (a situation similar to that of the positionerror described above).

The above structures allow the focus of the red and blue beam to bemeasured separately, greatly simplifying the adjustment of the focus ofall three beams.

In summary, the invention is related to a cathode ray tube of the indextype wherein the tracking structure allows the type to be used inprogressive scan mode. The tracking structure comprises trackingelements of a first kind 16 and a second kind 18 for generating a firstresponse signal S1 and a second response signal S2, respectively, whenhit by an electron beam of the tube, the first and the second responsesignals for determination of a positioning signal, and the trackingelements 16,18 extending parallel to the phosphor elements2000,2000′,2000″,2020,2020′,2020″, 2040,2040′,2040″ whereby eachphosphor element is flanked on either side by a tracking element of thefirst kind 16 and a tracking element of the second kind 18,respectively, except for each phosphor element of the third B color2040″ of each third set 330, whereby each side of said phosphor elementof the third B color is flanked by tracking elements of the same kind.This tracking structure has the advantage that the index tube has nonoticeable flicker if operated in interlaced scan mode.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. In the claims, any reference signsplaced between parentheses shall not be construed as limiting the claim.The word “comprising” does not exclude the presence of other elements orsteps than those listed in a claim. The word “a” or “an” preceding anelement does not exclude the presence of a plurality of such elements.

1. A cathode ray tube (1) of the index type comprising: a gun (6) forgenerating an electron beam (7,8,9), means (11) for deflecting theelectron beam (7,8,9) across an inner surface of a screen (10), responsemeans for controlling the deflection in response to a positioningsignal, the screen being provided with phosphor elements(2000,2000′,2000″,2020,2020′,2020″, 2040,2040′,2040″) for generatinglight when being excited by the electron beam, the phosphor elementsbeing grouped in sets of three phosphor elements (310,320,320) that emita first (R), a second (G) and a third color (B) of light, respectively,when excited, the screen (10) further being provided with trackingelements of a first kind (16) and a second kind (18) for generating afirst response signal (S1) and a second response signal (S2),respectively, when hit by the electron beam, the first and the secondresponse signals for determination of a positioning signal, and thetracking elements (16,18) extending parallel to the phosphor elementswhereby each phosphor element is flanked on either side by a trackingelement of the first kind (16) and a tracking element of the second kind(18), respectively, except for each phosphor element (2040″) of thethird (B) color of each third set (330), whereby each side of saidphosphor element of the third (B) color is flanked by tracking elementsof the same kind (16).
 2. A cathode ray tube of the index type accordingto claim 1, wherein a subset of the tracking elements (16,18) have gaps(30,30′) for deriving an additional positioning signal for positioningthe electron beam (7,8,9).
 3. A cathode ray tube of the index typeaccording to claim 1, wherein gaps (30,30′) of m adjacent phosphorelements form a first column (42) and gaps (31,31′) of n adjacentphosphor elements form a second column (44), both columns (42,44)extending in a direction perpendicular to the tracking elements (20),the first (42) and the second (44) column being positioned adjacent toeach other.
 4. A cathode ray tube of the index type according to claim3, wherein m is equal to nine and n is equal to five, while the first(42) and the second (44) column are positioned symmetrically withrespect to each other.
 5. A cathode ray tube of the index type accordingto claim 4, wherein the first and the second column form a T-structure,and the inner surface of the screen is provided with a set ofT-structures that are distributed over the screen according to thepositions of a x by y-matrix.
 6. A cathode ray tube of the index typeaccording to claim 5, wherein x and y are equal to nine.
 7. A televisionsystem provided with the cathode ray tube according to claim 1, whereinthe television system comprises means for providing a control signalbased on the first (S1) and second (S2) response signals.